Construction Guidelines for Plastic Inspection Wells: Construction Methods of Drop Manholes

According to CECS227:2007 Technical Specification for Application of Plastic Drainage Inspection Wells in Building Communities, a drop manhole base shall be adopted when the elevation difference between the upstream and downstream pipelines of an inspection well exceeds 2 meters.

A drop manhole refers to an inspection well where water flow drops inside the well. It is installed to meet design requirements when the elevation difference of drainage pipelines is large and the normal pipeline gradient fails to satisfy design criteria, featuring internal pipelines with a vertical drop.

Compared with conventional inspection wells, drop manholes are designed to dissipate the energy generated by falling water (the magnitude of such energy depends on water flow rate and pipeline elevation difference), thus requiring a more robust well base structure.

Layout Principles for Drop Manholes

A drop manhole is recommended when the vertical drop head of pipelines ranges from 1.0 m to 2.0 m; a drop manhole shall be installed when the vertical drop head exceeds 2.0 m, with the general maximum vertical drop head limited to 6 m. Drop manholes shall not be installed at pipeline bends. Where the diameter of the inlet pipe of a drop manhole is no greater than 200 mm, the single vertical drop head shall not exceed 6 m. For pipelines with diameters ranging from 300 mm to 600 mm, the single vertical drop head should not exceed 4 m. Vertical drop can be realized via vertical pipes or rectangular vertical chutes. For pipelines with diameters over 600 mm, the single vertical drop head and vertical drop mode shall be determined through hydraulic calculation.

Classification of Drop Manholes

Drop manholes fall into four main categories: brick-built drop manholes, modular block drop manholes, cast-in-place concrete drop manholes, and plastic drop manholes. The first three types are constructed on-site, featuring large floor space, long construction periods, reliance on construction machinery, and complex construction procedures.

Plastic drop manholes are integrally formed by extra-large injection molding machines, with flexible connections using rubber sealing rings on-site. They boast compact dimensions and no requirement for construction machinery, substantially cutting construction time and costs.

Two Construction Methods for Drop Manholes

Connect the drop manhole via additional shaft connectors (saddle joints) (see attached drawing).